A zero intermediate frequency (ZIF) tuner, also referred to herein as a direct frequency converter and single stage frequency converter, converts between relatively high frequency signal and baseband directly rather than through one or more intermediate stages or intermediate frequencies.
ZIF tuners may introduce imbalance between in-phase and quadrature phase (IQ) paths. IQ imbalance may arise from current leakage, characteristics, and differences between circuit components in I and Q paths. IQ imbalance may include frequency dependent IQ imbalance arising from low pass filters of a ZIF tuner, and frequency independent IQ imbalance arising from mixers of the ZIF tuner.
IQ imbalance may be reduced with stricter tolerances of ZIF tuner components and/or with compensation applied in an analog domain, but at a relatively substantial cost.
Where ZIF low pass filters have a relatively flat spectrum response and a linear phase over a passband, IQ imbalance may be compensated with gain and phase correction in a digital domain. A narrow bandwidth system may have a relatively flat low pass filter spectrum response and a relatively linear phase over a bandwidth. Thus, calibration and compensation may be performed in a digital domain. In wideband systems, however, such as a Multimedia over Coax Alliance (MoCA) standard based system, a ZIF tuner may have relatively high order low-pass filters to provide a wide passband and sharp cut-off. In such a wideband system, a flat passband may be prohibitively expensive and digital gain and phase compensation techniques may not be sufficient to correct IQ imbalance.
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